FIELD OF THE INVENTION
The invention relates to a fuel assembly of a pressurized water reactor, having a lower tie plate being permeable to coolant; an upper tie plate with an upper surface, a practically flat lower surface, and coolant openings; a bundle of fuel rods located next to one another, extending practically from the lower tie plate up to the upper tie plate; and a plurality of control rod guide rods, each having one bush on its upper end, being screwed to a can inserted into a retaining bore in the upper tie plate. The invention also relates to a fuel assembly of a light water reactor, having an end plate on one end being permeable to coolant; a cover plate on the other end, having an outer surface pointing outward, an inner surface pointing toward the fuel assembly interior, coolant openings, and at one retaining bore; a bundle of tubes being located next to one another with the majority of the tubes containing fuel; and at least one bush lengthening the end of a tube and being retained on the cover plate.
Fuel assemblies of pressurized water reactors include a lower tie plate that is permeable to the coolant, an upper tie plate with coolant openings, and a bundle or cluster of fuel rods located next to one another, with the cluster extending practically from the lower tie plate to the upper tie plate. Several of the regularly disposed fuel rod positions in the cluster are not occupied by fuel rods but rather by control rod guide tubes to which the lower tie plate and upper tie plate are secured, so that these plates form a stop for the fuel rods, which are guided through the holes in a plurality of gridlike spacers secured to the guide rods and are supported on grid ribs of the spacers. The guide rods are secured to the lower tie plate, for instance because the lower ends of the guide rods have threaded bolts that are passed through remaining bores in the lower tie plates and are screwed from below through nuts on the lower tie plate.
The invention pertains in particular to the fastening of the upper tie plate to the guide rods, for which purpose welded connections are unsuitable as a rule. This is because, although the upper tie plate is often made of austenitic steel, in modern pressurized water reactors, for the sake of neutron economy, guide rods of a zirconium alloy are used as a rule. Moreover, it is often necessary for purposes of inspection and repair to remove the upper tie plate and take out individual fuel rods from the fuel assembly. Releaseable connections are therefore desirable, yet they should include as few components as possible and the components should be secured against falling out.
Such screw fastenings often include one bush each, which is mounted (welded, for instance) to the upper end of the guide rod and screwed to a can that is inserted into a retaining bore in the upper tie plate and secured there.
In known screw fastenings, the bush of the guide rod has an internal thread for being screwed to the can inserted into the retaining bore of the upper tie plate and has a polygonal outside cross section, with which the bush protrudes into a corresponding polygonal recess on the lower surface of the upper tie plate, so that it can absorb the torque occurring when the bush is screwed to the can. The can is turned by its external thread far enough into the internal thread of the guide rod bush that an edge of the can, being formed as stop, comes to rest on the top of the upper tie plate, and by means of deformation the can rim can be pressed into a suitable profile on the top of the upper tie plate to secure the can against twisting and against unintentional loosening.
The polygonal recesses on the lower surface of the upper tie plate are complicated and expensive to produce, and they require a considerable minimum spacing on the part of the coolant openings in the upper tie plate, where the least possible pressure loss should occur, from the retaining bores that carry the can.
Moreover, for the length of the guide rods and the polygonal recesses on the lower surface of the upper tie plate, variations dictated by production requirements are unavoidable. It therefore may occur that when the cans are screwed together with the bushes, some guide rod bushes will be tightened until they meet the recesses of the upper tie plate, while the screw fastenings of other guide rod bushes have not been tightened up to a stop. All of the forces occurring between the guide rods and the upper tie plate are then absorbed only by the few screw fastenings that have been tightened up to a stop, while the other screw fastenings are seated loosely. That reduces the strength of the fuel assembly, and the heavily loaded screw fastenings can break. Moreover, there is no assurance that the upper tie plate will be aligned perpendicularly to the axis of the guide rods.
Provision is therefore made, in that known construction, not to weld the individual guide rods together with their bushes until the bushes have been turned into place and the upper tie plate has been aligned. When welding is then done, individually occurring deviations from standard dimensions can be compensated for. Therefore each upper tie plate is individually adapted to the individual fuel assembly, and in inspection work care must accordingly always be taken not to mistake the upper tie plates belonging to the various assemblies for one another. Moreover, care must even be taken to ensure that the same cans of the same upper tie plate will always be screwed together with the same bushes of the guide rods. This requires special monitoring in repair work.
In order to retain the guide rods on the lower tie plate of a pressurized water reactor as well, or to fasten fuel rods (particularly so-called "carrying rods" or "tie fuel rods", which are screwed to both the head and foot of the fuel assembly) and other tubes to a tie plate, a mechanically stable screw fastening is often sought that is easy to mount and easy to release, that includes only a few parts, which are prevented from falling out as much as possible, and which affords sufficient space for many and/or large coolant openings in the tie plate.
Accordingly, the invention provides--in a more generalized way--for retaining a tube on a cover plate which is located on one end of the fuel assembly and covers the bundle of fuel rods. This cover plate has an "inner surface" pointing toward the fuel rods, and an opposite "outer surface". A plate which covers that bundle at the opposite end of the assembly may be called "end plate". In the above mentioned case the tube is a control rod guide tube and is fastened on the upper tie plate; the "cover plate" and its "inner surface" stands for the upper tie plate and its lower surface, but in the more general case, the tube may be the cladding of a fuel rod, too. Additionally, the "cover plate" may also stand for the lower tie plate. In this case, the inner and outer surface of this cover plate is its upper and lower surface, respectively.
Published International Application WO 92/05565, corresponding to U.S. application Ser. No. 08/033,590, describes a boiling water fuel assembly with an end plate being permeable to coolant on one end (head), and a cover plate with coolant openings having an outside pointing outward and an inside pointing toward the interior of the fuel assembly. In that case, the two plates cover the two open ends of a fuel assembly case laterally surrounding a cluster of rods.
The rod bundle or cluster of said known fuel assembly includes tubes located next to one another, and the majority of the tubes contain fuel or in other words are fuel rods. At least one of the tubes is lengthened on one end by a bush that is retained on the lower tie plate ("cover plate"). This one tube is constructed as a coolant tube.
In other words, it carries liquid coolant from the foot into the upper part of the fuel assembly. Its bush has an external thread with which it is screwed into a retaining bore in said lower tie plate. A bolt is passed through the tie plate and the bush, and after assembly is welded to one of the parts and secures the screw fastening of the bush against twisting.
In order to release the screw fastening, the bolt must be destroyed, for instance by being drilled out or severed.
In the case of other tubes, such as fuel rods, that are held to the lower tie plate (cover plate), by one end, but extend over only a portion of the fuel assembly (known as "partial-length fuel rods"), bayonet mounts are provided for fastening purposes.
In order to fasten such coolant tubes and partial-length fuel rods as well, a screw fastening that is as rational, space-saving and stable as possible and that can be released only from the outside with a suitable tool, is advantageous.